CCELENTERATA. 



173 



serve also as organs of defense (Fig. 81). Digestion and 

 circulation both take place in a general cavity (gastro-vascu- 

 lar) lined with entoderm. In other words the circulatory 



FIG. 81. 



c 



-nu. 



FIG. 81. Nettling cells of Hydra (after Schmeil). A, unexploded; B, exploded, b, 

 barbs; c, the nettling cell in which the nettling organ is developed; en., the cnidocil or 

 "trigger"; cp., the capsule or nettling organ; f, the nettling filament or lasso; n, neck 

 of the capsule; nu., nucleus of the cell. 



Questions on the figure. Compare the parts of the nettling organ 

 before and after explosion and note the difference in position. How would 

 the barbs in the neck of the capsule behave as it is forced inside out by 

 the compression of the capsule? Find from your reference literature the 

 nature of the fluid secreted on the inside of the lasso. 



